Herbicide-resistant weeds

I recently attended the Pan American Weed Resistance Conference and it was excellent. It is always good to get perspectives from scientists both in our country and abroad. It is interesting that some countries are in about the same shape as we are when it comes to herbicide resistance, but some are better off.

Regardless of the crop or weed, the overall theme of the presentations was diversity, diversity and diversity. Unless we can break old habits and become more diverse in our cropping and herbicide systems, we are going to just chase one resistant weed after another and lose one herbicide mode of action after another.

Glyphosate resistance was the major topic of discussion. That is obviously a big issue in Arkansas as well as the United States in general. However, I am equally concerned in Arkansas about our herbicide situation in rice.

With the Clearfield acres being at least 50 percent and climbing, it is only a matter of time before that technology blows up. On the bright side, I thought it would already have blown up, so maybe I am wrong. However, can you afford to take the chance that I am wrong?

We already have widespread barnyardgrass resistance to propanil. While some growers may still be successfully controlling barnyardgrass with propanil, I never recommend propanil alone for barnyardgrass control. The reason is the resistance is so widespread, that unless I specifically know the field, I assume the population to be propanil-resistant.

While quinclorac (Facet and other quinclorac-containing herbicides) resistance is not as widespread, it is quite common — especially to postemergence applications of the herbicide.

Given the above situation, the herbicides we are putting the most selection pressure on right now are Command, the ACCase herbicides (Ricestar HT and Clincher) and the ALS herbicides (Newpath, Beyond and Regiment).

Resistance in the barnyardgrass family (Echinochloa) to the ACCase inhibiting herbicides is well-documented in California as well as in several Latin American countries. Ricestar HT and Clincher have become major components in many of our weed control programs in Arkansas. Fortunately we have not had major resistance issues with these herbicides to date. However, the University of Arkansas has tested two tighthead sprangletop populations from Louisiana that are highly resistant.

As the acreage in Clearfield rice continues to increase, we are increasing the selection pressure on the ALS inhibiting herbicides — Newpath and Beyond. In addition, Regiment is becoming increasingly popular and has become the major herbicide to control big barnyardgrass in conventional rice.

Junglerice (Echinochloa colonum) resistance to both the herbicides used in Clearfield Rice and to Regiment is well-documented in Latin America. In fact, two different mechanisms of resistance to these have been documented there. In addition, the scientist from Costa Rica reported they could document multiple resistance to propanil, the ACCase herbicides, the ALS herbicides and quinclorac.

The good news is we are still doing a remarkable job of weed control in Arkansas. The bad news is when the herbicides in the ACCase and ALS inhibiting groups blow up, they tend to do so in a hurry. We can not allow that to happen.

If you stop and think for a minute, in the big picture we are hanging our weed control future on one herbicide — clomazone (Command and now Alert). That is the herbicide that determines how our weed control program goes in most conventional fields. It is also the herbicide we are depending on to help preserve the Clearfield technology for barnyardgrass control. I will start here next time.